Why text object can't align to Y axis accurately?
Your location is based upon you origin (little orange dot)



If this is offset from your geometry then you will have problems in setting the exact location through this method. Also with text the origin will be the same location, but per character/font/style you may need it to be in a different location. There are some other nuances for example, here the default Bfont drops the lower case characters under the origin.



So you can set it up (by adjusting your origin), but this will not be automatically applied to each new text object. 

So in your specific example (where you have converted the text to a mesh) adjust the origin to the lower left of the P and it should just go to the world origin if you set the transform to X=0, Y=0. NB Moving Origins is easy now in object mode, press Options and select Origins and then you can simply move the origin of the object. I would recommend doing this by snapping to vertex.